Medical Imaging, in general, is the pictorial representation of the spatial distribution of one or more measured properties of a selected region of a patient being imaged. In x-ray computerized tomographic (CT) imaging devices, for example, the region is selected by directing the x-rays to a specific region of interest. The property measured by the x-ray devices is the x-ray absorption characteristic of the organs, tissue and bones in the section being images.
In NMR imaging systems, the region of interest of the patient's body is selected by a combination of a constant static magnetic field, one or more time dependent magnetic field gradients and a radio frequency pulse of a certain frequency and time dependent amplitude.
It is known that certain elements, such as hydrogen (the element most prevelant in the human body), passes nuclei with non-zero nuclear spins. Such nuclei exert magnetic moments which are normally randomly distributed. However materials containing such nuclei are macroscopically magnetized when a strong static field is applied which aligns the nuclear magnet moments in the direction of the static magnetic field.
When the aligned nuclear magnetic moments are subjected to an additional magnetic field that is perpendicular to the static field and rotates at a specified frequency, the nuclei will change their orientation with respect to the static field. This specified frequency is called Larmor's frequency and is defined by: EQU f=.gamma.HO/2TT
Where
f=frequency in Hertz PA1 .gamma.=a constant specific for the particular type of nucleus in Hertz/Gauss PA1 Ho=static magnetic field in Gauss
In practice the Larmor frequency is in the radio-frequency (RF) region. This perpendicular rotating field is applied in the form of pulses of certain temporal dependence which is the subject of this invention.
When the rotating magnetic field is removed, the nuclear spins:
(1) Start to precess around the direction of the static magnetic field; and
(2) Return slowly to their original orientation parallel with the static field.
This nuclear precession induces a voltage in a properly located receiving coil, which is amplified and processed. The resulting signal is called the Free Induction Decay (FID) signal. It is a measure of the density of the excited nuclei precessing within the detection area of the receiving coil.
By placing a patient in such a magnetic field Ho and applying RF pulses, signals from the patient's body can be detected.
In addition to the static magnetic field and the RF pulses, the patient is subjected to time dependent magnetic fields which:
(1) Are differently oriented with respect to the constant magnetic field when one or more gradients are applied during the RF pulse; and
(2) precess with different Larmor frequences when gradients are applied during FID signal observation.
Thus the gradients enable spatial resolution and define the location of different parts of the body in the received signal. Therefore it is possible to create images of the proton (hydrogen) distribution within the patient's body.
As is well known there are three basic relaxation times which are: (1) the spin-lattice or longitudinal relaxation time T1; (2) the spin-spin relaxation time T2; and (3) the decay time T*2 of the FID signal due to field inhomogeneities.
The longitudinal relaxation time T1 is the time constant determined by the time required for the nuclear magnetization to return to its original direction after the application of an RF pulse. That is the time it takes for the nuclei to reach equilibrium with their environment (lattice) after the RF pulse.
The spin-spin relaxation time T2 is the time constant determined by the time required for the nuclei that have been reoriented by the RF pulse from the original direction imposed by the static magnetic field to go out of phase with each other because of the effects of the neighboring spins.
The transverse relaxation time T*2 is the time constant determined by the decay of the FID signal due to field inhomogeneities including magnetic field gradients which cause the individual nuclear magnetic moments to precess at different frequencies and hence go out of phase.
There are many different operating procedures for acquiring the spin densities and the relaxation times within a selected region of the patient's body. Among the variables adjusted to provide different types of images are the time duration, shape and amplitude of the RF pulse. For example, the RF pulse can be applied so that the nuclear magnetization is rotated 90 degrees, 180 degrees, or any other amount from the direction of the static magnetic field. It is common practice in medical imaging to use sequences of 90 degrees and 180 degrees.
Another factor that is changed is the order of the sequences of the 90 degrees and 180 degrees RF pulses. Thus there are imaging methods wherein a 180 degrees pulse is used first and is followed by a 90 degree pulse. This sequence is known as inversion recovery. There is a method in which a 90 degree pulse is used first and subsequently a 180 degree pulse is applied. This sequence is known as spin-echo.
The spin-echo technique is ideally suited for obtaining accurate measurements of the T2 relaxation time in addition to the other characteristics that are provided by the NMR images. Note that in the spin-echo procedure the repeated applications of the 180 degree pulses result in repeated echoes. The repeated echoes lose amplitude at a rate determined by the time constant T2.
However, no matter which technique is used it is necessary to relate the measured spin densities to spatial locations. In practice the determination of particular locations is harder to achieve than in theory.
For example, it has been found that for pulses up to 90 degrees selective excitation of the imaged body is achieved by using a particular gradient during the application of the RF pulse. This procedure is called slice selection. The aim of this process is to excite slices with rectangular cross-sections. In theory the frequency of the applied RF pulse in conjunction with the field gradients excite only nuclei in the plane determined by field strength in accordance with the Larmor relationship. However, this statement is only an approximation: nuclei with Larmor frequency in the neighborhood of the applied RF pulse frequency are also excited. This deteriorates the slice porfile so that it no longer is rectangular.
There is no theory that tells how to excite an accurate rectangular slice of known width. However, people in the field usually use amplitude modulated RF pulses for this purpose. See, for example, L. E. Crooks, IEEE Transaction Nuclear Science, Vol. NS-27 (3), 1239, June 1980 and R. Sutherland, J. Huchinson, J. Physics E, Vol. 11 79 (1978). It was found that for RF pulses up to 90 degrees, a sinc (that is (sin 2.pi.t)/2.pi.t, where t is the time) shaped amplitude modulated RF pulse provides adequate selection of the plane to be imaged. However, when 180 degrees pulses are used, such as during a spin-echo sequence, the sinc shaped pulse does not provide the necessary selectivity. See P. R. Locher Phil. Trans. R. Soc London B 289 537 (1980).
The use of the various imaging techniques are described in patents such as U.S. Pat. Nos. 4,307,344, 4,397,337 and 4,070,611 for example as well as the patents, scientific articles, and books noted in those patents. Among the patents cited above are patents that relate to means and methods for selectively exciting prescribed sections of the subject being imaged.
In summary, the present techniques for obtaining data exclusively from rectangular shaped slices are inadequate. The present techniques do not exclusively excite rectangular shaped slices for 180 degrees RF pulses. Also a part of the observed signal is obtained erroneously from non-selected slices. Thus, there is a present and relatively long-standing need to provide RF pulses which will excite nuclear magnetic moments only in selected rectangular slices.